As is well known to those having skill in the art, monocrystalline silicon carbide is particularly well suited for use in semiconductor devices, such as integrated circuit semiconductor devices and power semiconductor devices. Integrated circuit semiconductor devices typically include many active devices such as transistors in a single semiconductor substrate. Power semiconductor devices, which may be integrated circuit devices, are semiconductor devices which carry large currents and support high voltages.
Silicon carbide has a wide bandgap, a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturated electron drift velocity compared to silicon, which is the most commonly used semiconductor material. These characteristics allow silicon carbide microelectronic devices to operate at high temperatures and higher power levels than conventional silicon based devices. In addition to the above advantages, silicon carbide power devices can operate with lower specific on resistance than conventional silicon power devices. The advantages of silicon carbide for forming power devices is described in a publication by M. Bhatnagar and coinventor B. J. Baliga entitled Analysis of Silicon Carbide Power Device Performance, ISPSD '91, Abstr. 8.3, pp 176-180 (1991).
Many microelectronic devices require the formation of trenches at a face of a semiconductor substrate. For example, one of the most popular types of power metal oxide field effect transistor (MOSFET) devices is the ultra-low on-resistance MOS device (UMOS). The UMOS device, also referred to as a "trench DMOS" device, is described in publications entitled An Ultra-Low On-Resistance Power MOSFET Fabricated by Using a Fully Self-Aligned Process, by Ueda et al., IEEE Transactions on Electron Devices, Vol. ED34, No. Apr. 4, 1987, pp. 926-930; Numerical and Experimental Comparison of 60V Vertical Double-Diffused MOSFETS and MOSFETS with a Trench-Gate Structure by Chang, Solid State Electronics, Vol. 32, No. 3, pp. 247-251, 1989; and Trench DMOS Transistor Technology for High-Current (100A Range) Switching by Buluce et al., Solid State Electronics, Vol. 34, No. 5, pp. 493-507, 1991. In forming a UMOS, reactive ion etching is typically used to form rectangular grooves or trenches in the substrate.
Unfortunately, while it is relatively easy to form well defined trenches having vertical walls, (i.e. walls which are orthogonal to the substrate face) in silicon substrates, using reactive ion etching or other techniques, it has heretofore been difficult to etch monocrystalline silicon carbide due to the inert nature of silicon carbide. Accordingly, although reactive ion etching of silicon carbide has been described, using various fluorinated gases such as carbon tetrafluoride and oxygen, sulfur hexafluoride and oxygen, or nitrogen trifluoride and oxygen, the results have not been entirely satisfactory. In particular, the etch rates using these reactive ion etching processes have not been as fast as is desirable for high volume, microelectronic device production, and it is difficult to form trenches with vertical walls.